It is well known that for many years railroad trains in North America, Europe, and other parts of the world have been equipped with some form of automatic pneumatic brake systems which are sometimes referred to as air brakes. The heretofore known strictly pneumatic brake system provides a simple, reliable and generally fail-safe means for permitting the engineer, conductor, or train crew to apply the brakes throughout the train. Such a pneumatic brake system includes an air compressor on the locomotive connected to a brake pipe extending throughout the train for providing compressed air to all of the cars and for operating brake cylinders on the cars which, through a rigging, apply braking action to the wheels of the cars. Pneumatic commands or signals are given by the engineer to the brake pipe which are sensed by control valves on each of the cars to apply braking functions. Generally speaking, when the reduction of air pressure is sensed in a brake pipe, brakes are applied, and when an increase in pressure is sensed, the brakes are released. Further, the degree of braking is in proportion to the value and rate of the reduction, although once brake pipe pressure begins to increase, the brakes are released completely.
A strictly pneumatic brake system has shortcomings particularly when used in a long freight train such as one having 150 cars being up to one and one-half miles long. For this length of train, it takes approximately eighteen seconds for the air pressure reduction initiated in the locomotive to reach the last car in the train. Accordingly, only a slight reduction in pressure is generally initiated by the engineer to prevent the last cars of the train from running into the cars with brakes applied strongly. Consequently, full pressure braking is delayed and braking distances are longer.
In order to solve the problems existing in strictly pneumatic braking systems, electronically controlled pneumatic brake systems (ECP systems) have been proposed in recent years and are currently being tested. For example, such a system is disclosed in U.S. Pat. Nos. 5,335,974; 5,722,736, and 5,924,774, all owned by the assignee of this application. It has been shown that ECP systems which incorporate a part of the automatic pneumatic brake system equipment on cars and controlled by a head-end unit (HEU) or master controller from the locomotive substantially improves braking and substantially reduces braking distances by providing substantially instantaneous and simultaneous brake signals to all of the cars so that they may brake at substantially the same time.
However, it is virtually impossible to completely retrofit all cars in existence at one time, and therefore a brake system capable of ECP operation must also be capable of strictly pneumatic operation, as more likely than not some of the cars on a train will not have the ECP system, and then control from the locomotive will be from a pneumatic controller.
It will be understood that full ECP systems require each car to be equipped with a car control device (CCD), and the locomotive with an HEU.